If <interval> is negative, the minimum number of video frames between buffer swaps is the absolute value of <interval>. In this case, if abs(<interval>) video frames have already passed from the previous swap when the swap is ready to be performed, the swap will occur without synchronization to a video frame. When an unsynchronized swap happens, the missed frame will count towards the minimum number of video frames for the next swap.

Maybe I'm having a "blonde moment", but what the heck does the above actually mean?